1). Field of the Invention
Embodiments of this invention relate to a junction field effect transistor (JFET) that provides greater control over current flow through the channel.
2). Discussion of Related Art
Semiconductor devices can be manufactured in the form of an integrated circuit or single device on a semiconductor substrate. A transistor is a type of semiconductor device that can be used for switching, amplification, signal modulation, and many other functions.
A type of transistor, called the field effect transistor (FET), relies on the application of a voltage to a gate in order to control the conductivity or current flow of a “channel.”
The channel region of any FET can be doped with either n-type implants or p-type implants, creating an n-type device or p-type device. Various types of FETs use different types of insulation between the channel and the gate.
Perhaps the most common FET is a metal oxide semiconductor field effect transistor (MOSFET) that uses an insulator between the channel and the gate, such as SiO2 (oxide).
Another type of FET, known as a JFET, utilizes a p-n junction as the gate. A conventional three-terminal JFET allows current to flow from a source to a drain while controlling the current flow with two gates.
Without a gate voltage, the charge carries flow in the channel region between the source and drain terminals and are “normally on” unless a gate voltage is applied. When the gate voltage is applied, a depletion region is created by pushing mobile carriers away from the channel and “pinching off” the channel.
Gate voltages can be varied to cause the JFET to act as a switch or to modulate the flow of current by affecting the cross-sectional area of the channel and the channel resistance. The type of JFET application will determine whether the JFET is most desirable as a switch or modulator.
In one example, JFETs can be useful in designing radio transceivers using direct conversion. Essentially, a radio frequency signal and local oscillator signal are fed into a mixer at the same carrier frequency. The signals are subtracted from one another, resulting in a low-frequency base-band output signal.
One of the problems with direct conversion is that the mixer must operate at very high frequencies while providing some gain, which introduces noise that makes signal processing difficult.
Mixer transistors should ideally be small in order to support frequencies in excess of 6 GHz. However, the area of the device is inversely proportional to the flicker noise created. At lower frequencies, the dominant flicker noise source in a MOSFET transistor is due to the interaction of the mobile charges with the silicon-oxide interface and the dopant ions in the channel.
In contrast, JFETs mitigate flicker noise, because the conduction occurs via the p-n junction, in the bulk, rather than near the surface of the oxide interface. However, a problem still exists with manufacturing JFETs with standard complementary metal oxide semi-conductor (CMOS) procedures. Manufacturing an effective JFET with standard CMOS procedures would have traditionally required carefully tailored implants to achieve the correct channel depth, which further requires additional masking, which increases the cost of the product. Many JFETs use a buried gate within the substrate material to act as another means to control the channel flow. If a buried gate is not used, the resulting JFET would inefficiently require up to several hundred volts to “pinch off” the channel.